PEPS (passive entry/passive start) systems are known in which the implementation of a function (such as unlocking the doors of a vehicle or starting such a vehicle) is conditional upon the presence of an identifier (generally carried by the user of the vehicle) in proximity to the vehicle.
Document US 2003/090 365 proposes measuring phases of reception of electromagnetic signals exchanged between the identifier and the vehicle for two different frequencies of the electromagnetic signals.
The distance separating the identifier and the vehicle may then be estimated on the basis of the difference between the measured phases.